1. Field of the Invention
The present invention relates to methods and apparatus for forming a subsea pipeline. More particularly, the present invention relates to methods for forming a joint in a subsea pipeline. Additionally, the present invention relates to methods of laying and positioning the subsea pipeline.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Offshore pipelines have normally been laid on the seabed by using a pipe laying barge. Pipe sections, typically 40 feet long, are welded to the end of the assembled pipeline on the barge and the pipe is launched over the stern of the barge as the vessel moves forward.
The part of the pipe between the vessel and the seabed adopts an S-shaped configuration or a J-shaped configuration having an upper curve called an “overbend” and a lower curve called a “sagbend”. It is important to ensure that there is not excess curvature in the overbend and the sagbend, or else the resulting high stress in the pipe can cause ovalization, buckling or fracture. Such buckles can be extremely expensive to repair. Typically, a supporting structure is employed to support the pipe in the overbend region so as to prevent excess curvature. Stingers are well known for this purpose and typically employ a buoyant structure for supporting the pipe. However, instead of a buoyant stringer, a fixed and rigid stern ramp is also known for supporting the pipe in the overbend. Such a stern ramp comprises a rigid structure extending from the stern of the pipe laying barge and remains fixed during pipe laying operations. The ramp is fitted with rollers along its length late which are positioned along an arcuate path for supporting pipe launched from the barge as it curves downwardly into the water in the overbend.
During these normal pipe laying operations, fresh pipe sections are welded to the end of the assembled pipeline on the barge, with the barge remaining stationary relative to the seabed, and similarly, the assembled pipe remaining stationary relative to the barge. When a fresh pipe length has been welded on and the joint finished as required, a length of pipe corresponding to the freshly added-on length is launched from the barge by moving the barge forward under the pipe and allowing the pipe to slide off the stern of the barge over the stern ramp or stinger.
During such pipe laying operations, it is very important to determine the touchdown point of the pipeline. The touchdown point is the point of which the pipeline contacts the seabed. In the past, ROVs have been employed for the purpose of determining this touchdown point. If the touchdown point is too close to the vessel, then there is a risk of buckling and/or overstressing. If the touchdown point is too far from the vessel, then there is a risk of the buckling of the pipe. As such, it is desirable to always ascertain the touchdown point of the pipe as the pipe laying operation continues.
ROVs have been employed for the purposes of determining this touchdown point. The ROV can utilize cameras in order to visually see the touchdown of the pipeline with the seabed. Unfortunately, the ROVs must be tethered to the vessel. In certain circumstances, the touchdown point may be nearly a mile from the vessel. As such, the length of the tether that is available may not be sufficient to cover such as distance. Under these circumstances, a second vessel, along with a second ROV, would be required. This significantly increases the expense of the pipe laying operation.
The efficiency of the pipe laying operation is largely determined by the efficiency with which the pipe can be welded end-to-end aboard the vessel. In the past, the pipes are welded in end-to-end relationship. Another section of pipe is placed over this welded joint and then welded to the pipe. This extra section of pipe can simply be placed over the pipeline in semi-cylindrical sections. These edges of the cylindrical sections are then welded together at the joint so that the pipe over lies the weld joint. This is a very time-consuming and inefficient operation.
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite welds are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. These pipelines extend through water that is thousands of feet deep, where temperatures of the water near the sea floor are in the range of 40° F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbons flow, any water present begin to cool, a phenomena that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines water at reduced temperatures so as to form a solid material, called a “hydrate”. Hydrates can plug pipeline. These plugs are very difficult to remove.
Typically, so as to avoid the effect of such low temperatures, the pipeline can be surrounded by an insulating material, such as concrete, insulating foam or electrical heating pipes. Since the insulating material extends along the pipeline, the section of the insulating material must be removed from the pipe so that the welding operation can occur at the pipe joint. As such, there will be a space, adjacent to the weld, that is free of the insulating material. There is a need to be able to suitably cover this exposed area after the welding operation has been completed. In certain circumstances, in the past, an insulating material is placed over the space. The application of this insulation material, in the past, has been very time-consuming. It often takes a great deal of time for the insulating material, such as epoxy, to effectively cure in this space.
In the past, various patents have issued relating to the forming of field joints for a subsea pipelines and relating to the method of laying and positioning a subsea pipeline. For example U.S. Pat. No. 3,690,111 issued on Sep. 12, 1972 to J. F. Matthews, Jr., describes an offshore pipeline installation method. The underwater pipeline is installed by lowering it to the bottom of the water from the stern of a lay barge as the barge advances along a long preassembled pipeline section which floats near the surface of the water and is held in tension by a second vessel positioned in front of the lay barge. An additional floating section is connected in place when the lay barge reaches the end of the initial section. This additional section is held in tension by the second vessel. Laying of the line is continued as the lay barge advances.
U.S. Pat. No. 4,120,167, issued on Oct. 17, 1978 to Denman et al., teaches offshore pipe laying in which a forward movement of a pipe-laying vessel is controlled to maintain the position of the pipe as laid on the sea bed. The position of the touchdown point on the sea bed of the pipeline suspended from the vessel is measured at periodic intervals by driving a survey vessel fitted with an echo location device along the already laid line. The measured position of a touchdown point is compared with the desired track and any deviation is computed. Further movements of the pipe-laying vessel are adapted to minimize this deviation.
U.S. Pat. No. 4,124,991, issued on Nov. 14, 1978 to W. M. Adler, provides an offshore pipe laying method which employs a pipe laying vessel with a fixed stern ramp and includes repeated steps of launching pipe while allowing pipe tension to drop within safe limits. Fresh pipe sections are welded on during the forward movements of the vessel.
U.S. Pat. No. 4,226,444, issued on Oct. 7, 1980 T. W. Bunyan, discloses a method of joining pipes in which a sleeve is placed over the adjacent ends of the pipes so as to overlap each pipe. The sleeve fits with clearance around the pipe ends. The ends of the clearance space are closed by inflating hollow sealing rings and then epoxy resin is injected into the clearance space to fill the space. The pressure of the epoxy resin composition is then raised to a pressure substantially greater than atmospheric and the pressure is maintained until the resin composition is set.
U.S. Pat. No. 5,328,648, issued on Jul. 12, 1994 to McBrien et al., shows a method of using a composite joint infill system. A pair of concrete coated pipe joints are welded together end-to-end with a gap between the concrete coatings. The gap is filled with a fast setting elastomeric polymeric infill material, either solid or foamed, and a particulate filler material. A mold is used for molding the infill material. The mold is filled with filler material before the polymer components are injected.
U.S. Pat. No. 6,058,979, issued on May 9, 2000 to L. W. Watkins, shows a deep sea insulated pipeline that has an inner pipe which is encased lengthwise by an insulating core. The insulating core comprises macrospheres surrounded by syntactic foam that includes a semi-rigid resin binder and microspheres. The semi-rigid resin binder reinforces the macrospheres to provide sufficient strength to withstand the hydrostatic pressure at depths in excess of several thousand feet of water, and is yet flexible enough to accommodate bending associated with deep sea pipe laying operations. The deep sea insulated pipeline may also include a protective outer casing. The inner pipe extends through and cooperates with the outer casing to define an annulus chamber containing the insulating core.
U.S. Pat. No. 6,641,330, issued on Nov. 4, 2003 to Andersen et al., discloses a method and apparatus for laying elongated articles. Fiber-reinforced flexible adhesive tape is used to bind an elongate article or bundle of articles during subsea laying operations. The apparatus includes at least one carrier for a tape spool arranged to rotate while moving bodily around the axis of the article during laying.
U.S. Pat. No. 6,739,803, issued on May 25, 2004 to Bass et al., teaches a method of insulating an electrically-heated pipe-in-pipe subsea pipeline Inner and outer pipe segments are formed and the inner pipe is coated and insulated. The coating may include sprayed polyurethane foam and insulating half-shells that are placed around welds. Epoxy is preferably coated on the inner pipe before other coatings. The segments are loaded on a lay barge and water stops are preferably installed in the annulus as the pipeline is formed. Water stops may be formed by placing a liquid polymer in the annulus and allowing it to cure.
It is an object of the present invention to provide a method that facilitates the accurate determination of a touchdown point of the pipeline from a remote distance.
It is another object of the present invention to provide a method that avoid buckling and overstressing of the pipeline.
It is another object of the present invention to provide a method which minimizes the ROV requirements.
It is still another object of the present invention to provide a method which minimizes the time required for forming field joints.
It is still another object of the present invention to provide a method that enhances the ability to monitor pipes and pipe joints.
It is still a further object of the present invention to provide a method which improves the buoyancy of the pipeline at the joints.
It is still another object of the present invention to provide a method that improves the insulating quality at the joints.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.